It is well known in the iron and steel industry that Mg metal is a useful inoculant for addition to molten ferrous metals; the Mg is known to be an effective desulfurizing agent for steel and is an effective nodularizing agent for preparing ductile iron.
It is also well known that Mg, as small particles, may be added to the molten ferrous metal by being carried through an lance by a stream of gas or in a carrier.
Mg metal, especially when in finely-divided form, is easily oxidized and is sometimes pyrophoric. In contact with water, it gives off H.sub.2 which, in ample quantities, presents an explosion or fire hazard. Various methods for reducing the pyrophoric and explosive hazards have been developed over the years and these developments have met with sufficient success to cause the iron and steel industry to remain interested in obtaining an economical, small particle Mg inoculant material which is relatively safe to store and use and which performs in a consistent, effective manner.
In the electrolytic production of magnesium by the electrolysis of molten MgCl.sub.2, it has been known for many years that the presence of boron values in the MgCl.sub.2 is detrimental to the complete coalescence of molten Mg formed during the electrolysis. It is known that seawater contains small amounts of boron and when seawater is treated with a alkaline material to precipitate Mg(OH).sub.2, a small amount of boron values may be also precipitated. Then when the Mg(OH).sub.2 is chlorinated to obtain MgCl.sub.2 for use as a feed material (also called "cell bath") to an electrolytic Mg cell, a detrimental amount of the boron values may accompany the MgCl.sub.2 unless steps are taken to remove, or at least substantially reduce, the amount of boron values. Thus, in the field of magnesium production, the attention given to boron values has been toward removing the boron values from the system. Even with such attempts made over the years to obtain substantially complete coalescence of molten Mg, formed in the fused salt electrolysis of MgCl.sub.2, so as to obtain a separable molten Mg phase, there is always some Mg which remains dispersed as droplets in the molten salt and in the cell sludge which is removed from the cell. When the cell sludge or the cell bath material is removed from the cell and freezes to a relatively hard (though friable) mass the small beads of Mg trapped therein in small quantities may be wasted unless there is provided an economical means for salvaging or utilizing the materials. Ordinarily the amount of Mg trapped in these frozen salt mixtures is only a small percentage of, say, less than 20%, usually less than about 15% by weight.
It is also known that in Mg alloying processes, e.g., the alloying of Mg and Al, the alloying is usually performed under a protective blanket of a molten salt flux. Some of the Mg alloy is retained in the flux material removed from the alloying process as a "slag". These alloying-process slags, somewhat similar to the frozen cell baths or cell sludges, contain small percentages of Mg alloy as discrete particles trapped therein.
In the past, efforts have been made to pulverize these matrices of sludges and slags into particle sizes suitable for commercial use as inoculants for molten ferrous melts, but because of batch-to-batch variations and the high salt content, the efforts had only limited success.
Also, there have been commercial efforts over the years to pulverize these sludges and slags to free the Mg particles from entrapment in the friable salt matrix and screen the particles from the salt or wash the water-soluble salts from the Mg particles. The Mg particles thus freed have been remelted for recovery and cast into ingots. The cost of obtaining such secondary Mg or Mg alloy in ingot form from sludges and slags requires comparison with the cost of primary Mg or Mg alloy ingots obtained from the principal sources, i.e., the electrolytic cell output and the alloying process output. Usually, if the market price of primary Mg or Mg alloy ingot is down because of decreased market demand, the recovery of secondary Mg or Mg alloy ingot from sludges or slags is not economical, so there is little or no incentive to perform the recovery.
However, we have found there are economical incentives for developing processes which will recover Mg or Mg alloy pellets from entrapment in sludges and slags (even though the pellets still contain a surface coating of the sludge or slag material) for use other than as casting into ingots. In fact, such pellets are useful as an inoculant material for molten ferrous melts and the protective salt coating is found to be beneficial, rather than detrimental.
The separation of solid Mg metal spheroids from entrapment in a solid contiguous matrix of a friable salt or mixture of salts presents particular problems to an investigator who may desire to recover the Mg in its spheroidal form and also retain on each spheroid a thin protective coating of the matrix material. Whereas it has been known for many years that such a Mg-containing matrix is removed as cell sludge from the electrolysis of molten MgCl.sub.2 and as a slag material from Mg or Mg-alloy casting operations, attempts to recover the Mg or Mg alloy particles by grinding or intensive ball-milling have generally resulted in smashing, breaking, or flattening a large portion of the Mg particles. Such deformed particles may be acceptable if the principal purpose of recovering the metal is that of re-melting it for coalescence or for re-casting as ingots.
In the present invention, however, what is of special interest is the recovery, from the solid matrix, of Mg spheroids which each have a thin protective coating of the matrix remaining. Such spheroidal Mg particles are of particular interest for use in inoculating molten ferrous metals, e.g., the desulfurization of steel. The thin protective coating of matrix helps avoid the hydrolysis of Mg by moisture or the oxidation of Mg by air. Mg particles which are substantially flattened or elongated or which do not have a high degree of rotundity are not as readily useful in operations where the particles are injected through a lance beneath the surface of molten iron or steel. Ideally, the operators of such lances would prefer that the Mg particles be of consistent size, consistent Mg content, and consistent rotundity in order to avoid unwelcome variances during the inoculation process.
The use of various grinding or pulverizing machines for reducing the particle size of various solid materials, such as rocks, ores and minerals, is well known. The use of screens or nests of screens to separate particles into various ranges of sizes is also well known. Very often the screens are vibrated to effect better, faster separations.
The separation of rotund beads from irregular shaped particles on a slanted surface is taught, e.g., in French Pat. No. 730,215; U.S. Pat. No. 1,976,974; U.S. Pat. No. 2,778,498; U.S. Pat. No. 2,658,616; and U.S. Pat. No. 3,464,550. A U.S. Department of Interior, Bureau of Mines publication R.I. 4286 dated May, 1948 on "New Dry Concentrating Equipment" contains information on a vibrating-deck mineral mineral shape separator; the separator disclosed is a vibrated tilted table where trajectory of particles across the surface is dependent on the shape of the particles. There are various sludges and slags from mining and metallurgical operations which are known to contain inclusions of metal droplets, such as copper, nickel, tin, and others.
U.S. Pat. No. 3,037,711 teaches the use of beater mills or hammer mills for pulverizing dross from metal particles, then separating the fines from the particles by suction.
General information about pulverizers, screens, and tabling may be found in, e.g., "Chemical Engineers Handbook" by Robt. H. Perry, Editor, published by McGraw-Hill.
U.S. Pat. Nos. 3,881,913 and 3,969,104 disclose the preparation of salt-coated Mg granules by an atomization technique and also disclose that such granules are useful for injection into molten iron through a lance.
Patents which teach the formation of small particles of Mg or Mg alloy on a spinning disc are, e.g., U.S. Pat. Nos. 2,699,576; 3,520,718; and 3,881,913.
The salt which may be employed in the present invention as the "matrix" material may be a single compound, such as a halide of Na, K, Li, Mg, Ca, Ba, Mn or Sr or may be a mixture of two or more of these salts. It is possible, and in some cases desirable, to employ mixtures of salts wherein the halide of one or more of the salts is a different halide than of the other salts. For instance, mixtures of MgCl.sub.2, NaCl, LiCl (or KCl), and CaF.sub.2 may be employed in various proportions. As used herein, the term "salt" comprises ingredients which are predominantly halide salts, but may also contain up to about 25% of oxides or other salts.
Various patents have described the molten salt mixtures, containing MgCl.sub.2, which may be employed in electrolytic cells for the electrolytic production of Mg metal, e.g., U.S. Pat. Nos. 2,888,389; 2,950,236; and 3,565,917. It is disclosed that the composition of the salt mixture may be varied in order to adjust the density to be greater than, or less than, molten Mg metal. Sludges formed in such electrolytic Mg processes are known to contain Mg metal particles entrapped in a matrix of salt, and, usually there are some Mg oxide values also present, due to contact with air or moisture. The use of fluorides in the salt mixtures as coalescing agents for the Mg metal is disclosed. Mixtures of salts are taught in U.S. Pat. No. 3,881,913 which are recognizable as mixtures such as are known to be employed in electrolytic Mg production as "cell bath" electrolyte compositions. Such cell bath compositions are also known to be present in Mg cell sludge and when the cell sludge is ground up to free the small beads of Mg metal trapped therein, some of the salt mixture is found to be present on the Mg beads as a thin coating.
At the 6th SDCE International die casting congress, organized by The Society of Die Casting Engineers, Inc. at Cleveland, Ohio on Nov. 16-19, 1970, there was a paper (Paper No. 101) on "Factors Controlling Melt Loss in Magnesium Die Casting", authored by J. N. Reding and S. C. Erickson. The paper discloses the entrapment of Mg particles and Mg alloy particles in sludges and slags, and discloses studies about coalescing agents and dispersion agents (emulsifiers) for the Mg particles. It also discloses the grinding, in a ball mill, of a Mg-containing sludge to recovery the Mg particles from entrapment therein.
Therefore, sludge material from Mg-producing processes, or from Mg-casting operations are known to contain Mg metal entrapped therein. In the Mg-producing processes, e.g., the electrolyzing of molten MgCl.sub.2 in the presence of other molten salts to produce Cl.sub.2 and molten Mg, the sludge material is composed of metal salts, oxides, impurities, and contaminants and contains a relatively small amount of Mg particles of various sizes dispersed therein.
During Mg casting, or Mg-alloy casting, the melt flux is usually provided on the surface of the molten metal in the melting vessel to prevent or retard contact of the metal with air or moisture and to prevent Mg fires. Such fluxes are usually mixtures of molten salts such as disclosed in U.S. Pat. No. 2,327,153 which also discloses the small Mg beads become trapped in the frozen sludge or slag as discrete fine globules having a diameter as small as 0.01 inch. The patent also discloses re-melting and stirring the sludge or slag in order to get the small Mg beads to coalesce into large beads of about 0.5 inch or large diameter, then partly cooling and separating the frozen beads from the still-molten salts by filtration.
Thus, the metal salt compositions of Mg cell sludges, Mg-casting slags, and Mg alloy-casting slags are a matter of record and are known to comprise various mixtures and ratios of alkaline metal salts, alkaline earth metal salts, some oxides and, generally, some impurities and contaminants.
An object of the present invention is to recover rotund, salt-coated Mg particles, or Mg alloy particles, from entrapment in a contiguous, friable matrix of sludge or slag material.
Another object of the present invention is the preparation of a contiguous, friable, matrix of salt material containing dispersed therein discrete, rotund particles of Mg or Mg alloy.
A further object is to recover such rotund, salt-coated particles by a process which substantially avoids flattening, rupturing, or pulverizing said particles.
Another object is to recover such rotund Mg particles in a manner that the Mg particles retain only a thin protective coating of the sludge material in which they were entrapped.
Yet another object is to recover such coated Mg particles with a relatively consistent Mg content and relatively consistent particle size range and rotundity for use as an inoculant through a lance into a molten ferrous metal.
These, and other beneficial objects, believed to be apparent to practitioners of the relevant arts, are substantially attained by the presently disclosed invention.